Seismic prospecting



Nov. 10, 1953 C. W. OLIPH ANT SEISMIC PROSPECTING Filed Oct. 5, 1948 PARTICLE MOTION IN LONGITUDINAL WAVE Particle P Direction of Ray Path 3 Sheets-Sheet l PARTICLE MOTION IN TRANSVERSE WAVES OF THE SV AND SH TYPES Particle P Direction of Ray Path GUIDE TO RECORD CHARACTER SOUTH Three Component NORTH Geophone RECORD CHARACTER FOR LONGlTUDlNAL PULSE Transverse Trace A Y Ground Moves East i Vertical Trace Ground Moves Up Longitudinal Trace Ground Moves South RECORD CHARACTER FOR TRANSVERSE PULSE OF THE SV TYPE Transverse Trace ,f Ground Moves East I Vertical Trace Ground Moves Up Longitudinal Trace Ground Moves South FIG. 5

#vvswrae CHARLES W- Ol-IPHANT ATTO/ENE Y5 Nov. 10, 1953 c. w. OLIPHANT 2,658,578

' SEISMIC PROSPECTING Filed Oct. 5, 1948 3 Sheets-Sheet 2 I Q0 1.0 I780 D I I I I I l snouun MOVES EAST GROUND MOVES SOUTH /N VE N T'OE ARLES W 01. FH v7- ar MJJ Z ATTOENEYS Nov. 10, 1953 c. w. OLIPHANT 2,658,578

SEISMIC PROSPECTING Filed 001;. 5, 1948 3 Sheets-Sheet 5 ./N VE N TOE CHARLES WOLIPHANT 7 Tana...

ATTORNEY:

Patented Nov. 10, 1953 UNITED STATES PATENT OFFICE SEISMIC PROSPECTING Charles W. Oliphant, Tulsa, Okla.

Application October 5, 1948, Serial N0. 52,858

3 Claims. 1

. The present invention is concerned with seismic prospecting, and, particularly, with methods of ascertaining the constitution and geometry of earth substrata by means of both longitudinal and transverse elastic body waves.

In general, the seismic refraction and reflection techniques, presently available in the field of exploration geophysics, rely on longitudinal elastic body waves. Such waves are generated by the detonation of a small charge of explosive in a shallow hole, and are known as direct waves because their origin is in the vicinity of the exploding charge.

In marked contrast, few attempts have been made to utilize transverse elastic body waves for seismic prospecting. Generally, the present practice is to ignore or, in fact, take steps to prevent the generation or recording of these waves.

The few cases attempting to utilize transverse elastic body waves, have relied largely on the generation of these waves, not at the shot point, but at some remote subsurface discontinuity where incident longitudinal waves are reflected and refracted, and, in this process, loose some small fraction of their energy to the generation of transverse waves; such waves are known as transformed waves because their origin is due to a secondary process in some region remote from the exploding charge. To obtain useful geologic information, in this instance, an atempt is made first to identify the subsurface region in Which the transverse waves were generated and, second, to trace out the shortest-time paths subsequently followed by these transformed waves.

From field studies in this connection, I have verified the fact that useful geologic information can be obtained from an analysis of the traveltimes of transformed transverse elastic body waves. Generally, however, the usefulness of the method is restricted to those areas where the geologic and seismic conditions are exceedingly simple. In areas of even slight complication, considerableuncertainty will exist as to the region of generation and subsequent shortest-time transmission path. Furthermore, it was found that the discontinuity in elastic parameters of 'earth substrata often does not favor the transformation to significant quantities of transverse wave energy; in this case the method fails because smallv quantities of transformed transverse energy are confused or obliterated by extraneous ground unrest.

My invention thus is not primarily concerned with transformed transverse elastic body waves.

. On the contrary, my-invention is directed toward the methods of utilizing both direct longitudinal and direct transverse elastic body waves, generated in the immediate vicinity of an exploding charge in order to ascertain the constitution and geometry of earth substrata. In the discussion below it will be shown first: that sound theoretical considerations predict the generation of both of these wave types at the shot, and second: that practical field methods and observations substantiate the theory. The result is a new and useful improvement in the method of seismic prospecting.

The present invention may be more readily understood from the following explanation and description of the accompanying drawings.

In Figure 1, the particle motion relative to a horizontal ray path is shown for plane longitudinal waves. The particleis displaced in a direction parallel to the ray path.

In Figure 2, the particle motion" relative to a horizontal ray path is shown for plane transverse waves of the SV and SH type. The particle in Figure 2 is displaced along some preferred line, in the plane perpendicular to the ray path.

In Figure 3 is a representation of earth substrata possessing different velocity character with a northbound wave from a detonation at a position south of the geophone approaching a three component geophone, representative of such geophones positioned at appropriately spaced receiving points. The orientation conditions of Figure 3 were chosen to fit the orientation conditions of the actual records for a three component seismograph shown in Figure '7 and enlarged in Figure 6.

In Figure 4 there-is illustrated an example of the record character to be expected from plane longitudinal waves. The direction of wave travel, wave path, and geophone orientation were chosen to fit the field conditions of the actual field records shown in Figures 6 and 7.

In Figure 5 there is illustrated an example of the record character to be expected from transverse waves of the SV-type. The direction of Wave travel, wave path, and geophone orientation were chosen to fit the field conditions of the actual field records shownin Figures 6 and 7.

Figure 6 is an enlarged view of the lefthand portion of Figure 7. p

Figure '7 is a record of actual seismograms obeters at appropriately spaced points, the three components of ground motion so produced, and

acsasvs 3 the recording of these motions and the shot instant as a function of time.

For the purposes of a theoretical discussion, attention is directed to a homogeneous elastic solid body in which plane waves are to be generated by an arbitrary disturbance; by a plane wave it is meant that the displacement, velocity or acceleration which the wave imparts to some very small volume ur itor particle ofthe material at any instant depends only on the time and dis-. tance from a fixed plane. Establishing, then, a coordinate system in which the axis of X is perl-E pendicular to this fixed plane and the axes of Y and Z are in it, and taking the respective displacement components parallel to these @XQSJflS. u, v, and w, the equations of mgtion become:

where: A and u;the elastic constants of the material Du v 61p riefs; =the density of the material t=.tirne A solution may be obtained by differentiating the three equations of (1) which, on consideration of the plane wave front, gives first for the equations of motion in the a: direction:

the general solution of Equations 2, 3 and 4 are:

where f and I may be any functions whatever. l'hus Equation 7 states, among other things, that displacement components of the ori inal disturbance parallel to the X axis will travel in the at direction with a velocity V in contrast. Equations 8 and 9 state that displacements perpendicular to the X axis will travel in the a: direction with a velocity Vs. Evidently, then, the arbitrary disturbances generates two wave types which travel with different velocity. v For the wave type represented byEquation '7 v and w are zero and any small volume unit of the disturbed material experiences compression which changes its volume but not its shape; these waves are known as direct longitudinal, compressional, dilational, or primary waves.

For the wave type represented by Equations 8 and 9 u is zero and any small volume unit of the disturbed material experiences a change in shape but not volume; these waves are known as direct transverse, shear, or secondary waves. ,In the case of these transverse waves the further polarized transverse wave, known as an SV wave: a horizontally polarized, SH wave, would correspond to the case where both u and u were zero.

These results are illustrated in Figures 1 and 2, where particle displacements, relative to the same ray path, are shown for each of the wave type predicted by Equations 7, 8 and 9. In the instance of a. lane longitudinal wave the particle is displace n a tion' par llel to. the ray path; for transverse waves the particle is displaced along some preferred line in the plane perpendicular to the ray path. The transverse wave may be termed SV or SH when the prein-case the 'wave fronts are spherical, there dist nction ma e ad hatw n b th it nd are zero, Equation 8 represents a vertically ferregl linear direction of polarization is respectively vertical or horizontal.

would be; no change in the particle motion of the longitudinal wave; the displacements in the transverse wave, however, would follow the curvature of the wave front and produce a rotational motion. The magnitude of such rotations would be exceedingly small, in cases of practical interest; evidently then, this second order rotational effect is ill-suited for the purpose of receiving and recording transverse elastic body waves. I

Thus, in summary, it has been shown from theoretical considerations that an arbitrary disturbance in a homogeneous elastic solid initiates longitudinal elasticbody waves which travel with the velocity .VP; I p

andin'addition, transverse elastic body waves which travel with the velocity The particle motion excited by the passage of these two wave types is decidedly different, and this latter fact is the proper criterion for dis,- tinguis hing one from the other.

Now, in comparing the theoretical predictions with actual field observations, I have found that the detonation of a small dynamite charge in a shallow, cylindrical hole ,will generate direct longitudinal elastic body waves as well as direct transverse elastic body waves of the SV type. The energy of the latter wave is comparable to the more frequently used and better known Ion-.- gitudinal wave; SH type waves have also been pbserved, but there is evidence that they did not originate at the shot point.

These observations infer that the generating motion of the source is not completely arbitrary. T pre sure fr'cm the exp ing charge acts radially on the sensor the cylindrical hole to produce a compression and thus initiate the direct longitudinal elastic body waves. The expandin'g gases of 'thedetonated charge also produce a compression'at the base of the cylindrical hole,- but since this compressional stress is acting at right angles "to the compressional stress Qdirecte'd radially to the walls ofthe cylindrical hole and'sijnce, in general, the stresses will not be of equal magnitude in all directions, a zone of shearing strain "existsabout the edges of the 'eircular base of thecylindrical hole. Apparently, then, the asymmetry of the stress distributionfavors the generation of'direct SV type tra sverse elas ic ody wa e body waves generated in the immediate vicinity of the shot point, significant improvements are eflected in the methods for investigating the constitution and geometry of earth substrata. The method comprises the generation of these wave types; the reception, at a series of appropriately spaced points, of the three principal components of ground motion which these waves produce; the recording of this motion and the shot instant as a function of time; and the analysis of recorded data for wave type, wave path and wave velocity.

The details of the method may be clarified by a discussion of the various necessary steps. First, in receiving, at a series of appropriately spaced points, the ground motion produced by longitudinal and transverse elastic body waves, it is imperative that the motion of a particle, disturbed by the passage of these waves, be completely described as a function of time. Complete description of the particle motion is only obtained when the three principal or mutually perpendicular components of its displacement, velocity or acceleration are measured, at a particular point; this is accomplished by a three component seismometer, for example, one consisting of three inertia-mass and spring systems arranged to respond only to motions in the three mutually perpendicular directions, corresponding to the principal axes X, Y and Z of the previous theoretical discussion. A single three component seismometer is required for the reception of ground motion at each point.

For a wave proceeding in the x direction, received and recorded particle motion when parallel to the a: direction is known as longitudinal motion; when parallel to the y direction, vertical motion; and when parallel to the a direction transverse motion. It will be shown that a longitudinal elastic body wave proceeding in the a: direction, can produce components of particle motion in the :r and y directions, and that the same is true of an SV type transverse wave. In this case, an SH type transverse wave would produce motion only in the z direction. The discus- I sion below will describe, in detail, the method by which plane longitudinal and plane SV type transverse elastic body waves are identified on three component seismograph records.

When wave paths are short and when transverse and longitudinal waves arrive almost simultaneously, the resultant ground motion is exceedingly complex; the confused character of the rec.- ords makes it impractical to attempt a distince tion between the two wave types on the basis of particle motion. At greater spread distances, the situation is more favorable because transverse waves, traveling with a velocity considerably lower than the longitudinal waves, arrive at, a later time. The distinctive motion inherent in each wave type is then adequate for identifying longitudinal and transverse elastic body waves. This motion is most easily understood by examining the particle displacement of each wave type relative to the same ray path.

Referring to Figures 4 and 5, a hypothetical pulse of pure longitudinal motion and pure trans-. verse motion of the SV type are applied to the conditions shown in the other figures. The traces shown in Figures 4 and 5 show the expected rec-. ord character. In a more realistic case, the mo-v tion of the two pulses would be recorded on one record and separated by a time interval governed by path distanceand the ratio of longitudinal and transverse velocities.

Two important facts should be evident from a comparison of these hypothetical records:

The largest motion for a steeply emergent longitudinal pulse is on the vertical trace whereas the largest motion for a steeply emergent SV pulse is on the longitudinal trace.

The directions of trace displacement are opposite for the longitudinal pulse (where a vertical trace peak occurs simultaneously with a longitudinal trace trough) whereas the directions of trace displacement are the same for the SV pulse (where a vertical trace trough occurs simultaneously with a longitudinal trace trough).

The latter characteristic was found to be the most useful. It should be noted that a change in the conditions of the example produces changes in the expected record character; for instance, if all conditions remained the same except that the waves were southbound, the longitudinal pulse would produce a vertical trace peak simultaneously with a longitudinal trace peak whereas a trough and peak would coincide for the SV pulse.

Attention is now directed to the seismograms of Figure '7 and the enlargement of its initial parts, Figure 6. These seismograms were obtained in the field by detonating a small charge of dynamite in a shallow cylindrical hole, receiving, by means of three component seismometers at appropriately spaced points, the three principal components of ground motion so produced, and the recording of these motions and the shot instant as a function of time. Since the seismometers were north of the seismic disturbance or shot point, see Figure 3, the recorded waves are termed northbound'and correspond to waves traveling in the positive a: direction; the horizontal distance from shot point to seismometer was varied from 1780 feet to 2395 feet at approximately foot intervals; the top trace of each seismogram records transverse motion parallel to the z direction with upward trace displacement corresponding to eastward ground movement; the second trace records vertical motion parallel to the y direction with upward trace displacement corresponding to upward ground motion; the third trace records longitudinal motion parallel to the x direction with upward trace displacement corresponding to southward ground motion; the fourth trace records the shot instant. Small vertical lines above each seismogram mark elapsed time after the shot instant in intervals of 0.1 second.

Figure 6 is an enlargement of the ground motions recorded on Figure 7 in the time interval between the shot instant and 1.0 second later.

Figures 6 and '7, particularly the enlargement shown in Figure 6, may now be compared with the representations in Figures l and 5 since'the orientation conditions of each are identical. From this comparison it will be seen that the vertical and longitudinal traces of the actual field seismograms exhibit'particle motions attributable to both longitudinal and SV type transverse waves. As expected, the field records do not achieve the idealized simplicity of Figures 4 and 5. The approach is sufilciently close, however, that the identification principle is applicable, and accordingly, on any oneseismogram, a particular longitudinal wave has been marked as P2 and the corresponding SV type transverse wave as S2, see the reference vertical broken line. Other longitudinal and transverse wavesare also recordedat difierent arrivaltimes, and could be similarly identified; point D, 'for.'example, onthe I I i I i wa e with 'eaiiierentena l I are id' may the shortest ti i or mo ern sp m to 3 2 properly analyze Qwavepath;

sen-n3 another longitudinal mater P2;

v i 7 7 Thus, wave] types be identified: it has been shown: that longitudinal; andjtransverse,

@de'rgeing 'co'nti-nuous "refraction by a; medium; I, n er trave ne ntnan I i v 1 dreams or a r'ef 'racted; wave :path; through-layers I with a discontinuous; increase of, velocity or in I waves may be distinguished and identified ivhen I I I the three principalcomponents of QIOQEHaQmOi'IibIL v which they produce are received by a threeoom-w I ponent seismometer and recorded, asaiunetion I I cal or the velocity with-Which theev'ent traveled of tirnaatappropriately spacedrpoints. In'contrast, it should be quite clear I that although a single componentseismometer is, capable ofi-re-v I I ceiving both I longitudinal and, transverse type I "types;

the twmva pio aeationveioc is" I y aha f' I observe thetinie of a1,

I I "-I'he theory and practice of travel'tiine, graph constmcticn andinterpi-tation: has been well es I vious; practices; however;

cussion.

i have found, 'for example, that the travel-time for both 'longitiidinal and SV type transverse elastic body waves, when :traced to shorter and shorter distance intervals between shot and recording points, must pass through the origin of the "navei time graph; that 'is to say, the travel time for thesejtwo waves 'decr'eases'with decreasing' tra-vel 'oista-hoe in such a way as to become zero a't 'zei'o s'paratidneiste nceor shot'and seisfn'o'mt'er. t has thus been verified "expertmentally that these wave types originate at the shot point. In the subsequent identification'of wavepaths; considerable b'ehiit derives from this known origin 'of both wave types, and 'much of the uncertainty inheren -m m analysis' of'trans formed wave paths is eliminated.

For purposes of illustration, and 'for thesepurposes alone, it is usefulto review thepropertiesflof a travel time graph. If'the arrival time o'i'ap'a rticular event recorded on a seismo'gram at varying distances'from the shot point is plottedonla graph whose abscissa'is distance andwhose ord mate is time, a travel-time lin'e i's'defineo. The properties of this line identify the'waye I If the line is straight the transmitting medium has a constant velocity andthe'wa'v'e'has traveled directly from shot to 'seismometercr has refracted at some subsurface discontinuity w th a vertical depth dependent on the time'intercept at zero distance. A curved travel-time'line cehcave upwards (towards'increasing values of time) must be that of a'refie'cted wave, whilea curved line, concave downwards (toward I decrea ing values 6: tiine) "isindicative-ofadirot Wa'v'e' unge in a shallow cylindrical; holefig; Other {dif irenes iwjill tbeeon'ije' apparent from g' further diswaves; the observationof but a single component, I

:tablished 'm prior operationsoi thearto fseisinic q seismic'inetnoa mierractiqn techniq a' r;: I examp e; the conventionalsmglecomponent seis- I I j j mo'meter, that is :31]; ineltiag-fmass and spring5y v I l v i whose velocity increases continuously with aleipth;- I I i A broken, 'but cohtinuouetthvel-tiine line is inrare instances of faulting :A truly discontinuous p f travemii ie'nne-is indie tive of a wave path ina hes ope as the total delay time,f-is indicative'ofthe depth atfw-h i'chthe wave was reflected or refracted.

; v 1 I v v In the construction fo'f 'tr'aveltinie graphs ior I I 1 emaimne steps; in the m mes-com rise the, purpose ofidentir in'g wave paths it should am ronowen by both 1 the determination ;I of j be clearly understood first that; [the arrival time partioular separationidistance of shot anti seisng as: low veioeity layerfbetween two h e- I 'loc tylayrS. y, I ,I

f the traventime line is the r'ecipi-oe I 1 I I 1 1 I mometer mustbjecorrelatedwith the arrival time i f i i a of the same significant eventfat a diflerentsep I I l I I ara'tion distance,- and second that: if the event I ;plottedonthetraveletime grapndoesnotrepree Z sentthetruefirstarrivaiofenergyoversomepg z I path,thenithepathwill bejerroneouslyidentified; Theseconsiderations;areperfectlyigeneralfandfg; i

apply equally to seismic refraction :or reflection I I i 1 techniques ponent seismoineters, it is usually true that the recorded reflection events which can be correlated ffrom oneseparation distance to another do not represent the true first arrival of energy over the permissible reflection paths.

With a three component seismoineter, that is a single base "plate to which is secured three i'nertia mass'and spring systems each responsive to motions in only one of the three principal directions, I have found. that the above limitations can more frequently be overcome. The recorded combination of three mutually perpendicular components of ground motion greatly increases the opportunities for correlating the same ground motion event at different separatioh distances of shot and seismome'ter, and, "in addition, frequently permits the determination of the true beginning of a particular event. In the geologic and seismic "field conditions where these operations are feasible, the absolute rather than 'therelativ'edepth of several-refracting or reflecting horizons may be obtained. Thus, by the' metho'ds of my invention, travel time graphs may be constructed for both'theclirect longitudinal elastic body waves "and the direct'transvers'eelastic body waves which-havebeen generated by an exploding charge in a shallow cylindrical hole. Because theinformation plotted on such agraphis'obtained from three component seis'mo'g'rams rather than the conventional single component seismogramitis more frequently possibleto oorrelatefthesameground motion event andto determine the true beginningat each of the :difierent separation distanc's of shot and seismometer. l

: i prioroperations of the art 1 these two re! j rements have limited the usefulness oi the I i I temrespon ive I to pmotiQn in; 'only one I of the I I I incipal directions; 'seldom'p'roduces seis I a a i v j I l moerams in; which events other: "than the first l I I I re'corded, motion canj b'ehorrel'ated at: different I I separation; distnces I of vshot and :sei's'rnometer; I reflection techniques, also utilizing single corn and from Equations 3 and 4 the propagation velocity of transverse waves was given by Equation 6:

Now, as previously discussed, the slope of the travel-time line is the reciprocal of the transmission velocity for the particular event or wave type under consideration. Thus, when similar wave paths have been traversed, in the same material, by both longitudinal and transverse elastic body waves and when the bulk density of that material is independently known, the

elastic parameters A and a of the material may be calculated. Additionally, it will be clear that other elastic parameters such as Youngs modulus, Poissons ratio, compressibility, etc., may also be calculated when VP, Vs, and p are known.

It should be pointed out that the above calculations rely on the assumption that the material in question is uniformly homogeneous and isotropic. In this regard, I have found that substrata, in general, do not achieve these idealized conditions. For this and other reasons, then, the constitution of earth substrata deduced in this manner is not sharply diagnostic. Major sedimentary and igneous rock or substratum types can usually be distinguished, as for example, a shale from a limestone or a granite from a gabbro, but the transitional lithologies, intermediate between the major types must remain uncertain.

In using this method to ascertain the constitution of earth substrata, significant advantages accrue from use of the direct longitudinal elastic body wave and the direct transverse elastic body wave. Since both waves originate in the immediate vicinity of the shot point, the

respective shortest-time wave paths, either direct, reflected or refracted, between shot or seism disturbance location and seismometer may be conveniently deduced from travel-time graphs. In this connection, I have found that direct or refracted wave paths, generally, are the most useful ones for ascertaining the constitution of earth substrata, since with direct or refracted wave paths the respective propagation velocities for both wave types are determined by a single stratum or layer. When reflected wave paths, and particularly when transformed reflected wave paths are involved, the propagation velocities for both wave types are determined not by a single stratum or layer but by all of the strata or layers intervening between the shot or disturbance point and the reflecting discontinuity; elastic parameters for reflected wave paths will thus have values influenced by the thickness of each different stratum or layer of the reflection path. In most geological circumstances the elastic parameters from a reflection wave path are useless for diagnosing the constitution of the material.

From the specification, it is apparent that my invention has wide application in the determination of the physical properties of the earths strata and that it is not limited to the particular type of equipment or arrangement of equipment referred to in this specification.

I claim as my invention:

1. A method for the investigation of earth substrata which comprises establishing a single disturbance point in the area to be investigated, creating a seismic single disturbance at the disturbance point which includes the generation of longitudinal and transverse body waves, establishing a reception point for seismic waves at each of a plurality of spaced points removed from the single disturbance point but sufiiciently close to the single disturbance point to receive energy from said disturbance point, and receiving at each reception point and recording as a function of time as separate indicia on a record the three principal and mutually perpendicular components of translational ground motion which said waves produce, together with the instant of their generation, said indicia completely describing the movements of each reception point in space as a function of time and said indicia being different in phase and amplitude for said longitudinal and said transverse body waves and serving to distinguish and identify them.

2. A method according to claim 1 wherein said reception of the three principal and mutually perpendicular components of ground motion are received and recorded as a function of time so as to produce a seismogram showing the difference in phase, amplitude and frequency of waves impressed on the reception point, and by the diiference in phase, amplitude and frequency distinguishing these waves and their arrival time.

3. A method according to claim 1 including the determination of the bulk density of said earth substrata, using said recordings of the ground motions for a determination of the propagation velocities of said longitudinal and transverse elastic body Waves proceeding along similar transmission paths in said earth substrata, and using said bulk density and said determined propagation velocities to measure the elastic parameters of said earth substrata.

CHARLES W. OLIPHANT.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,046,104 Blau June 30, 1936 2,216,452 Owen Oct. 1, 1940 2,354,548 Ricker July 25, 1944 2,390,187 Rogers Dec. 4, 1945 2,482,233 Arringdale Sept. 20, 1949 2,555,806 Mitchell, Jr. June 5, 1951 2,576,775 Case Nov. 27, 1951 

